1. Technical Field
The present invention relates to an adjuster assembly for compensating for brake lining wear in a brake unit. The invention further relates to a brake unit comprising an adjuster assembly, as well as to a vehicle equipped with a brake unit comprising an adjuster assembly.
2. Background of the Invention
Braking systems for heavy goods vehicles are presently generally powered by compressed air. A typical braking system comprises a brake unit mounted on a hub of a wheel to be braked. A pneumatic actuator is arranged on the brake unit such that the actuator acts on a brake lever pivotally supported in the brake unit. Displacement of the brake lever causes friction linings to come into contact with a brake surface arranged on a component which rotates with the wheel to be braked. In an attempt to ensure reliable operation of braking systems, legislation dictates a maximum permissible displacement of the brake lever, i.e. the maximum stroke of the lever, to still obtain maximum braking effect.
When new friction linings are fitted to a brake unit, the unit is adjusted so that a certain clearance is attained between the friction linings and the brake surface. In other words, a predetermined stroke of the brake lever will result in a maximum braking effect. Naturally, as the friction linings wear during use, the stroke of the brake lever will increase if the same braking effect is required. Once a certain degree of wear has taken place, the stroke can exceed the legislated maximum amount.
Accordingly, it is known to provide braking systems with means to compensate for friction lining wear, hereinafter termed an adjuster assembly, to thereby ensure that the stroke of the brake lever is always maintained within a predetermined length during the entire useful working life of the friction linings.
Known adjuster assemblies employ mechanical sensing means to determine the stroke of the brake lever. If the stroke exceeds a predetermined amount, the sensing means causes a take-up means to rotate to ensure that, in a released position, the brake linings sit closer to the brake surface than before rotation of the take-up means. To prevent the brake linings reverting to their original position when the brake lever is released, the sensing means communicates with the take-up means via a one-way clutch assembly so that the take-up means can only rotate in a direction which takes up excess play in the system.
When the brake linings have exceeded their useful working life, they must be removed from the brake unit and new linings fitted thereto. This implies that the adjuster assembly must be xe2x80x9creleasedxe2x80x9d so that sufficient clearance is attained to allow the new linings to be fitted. In one braking system, this is achieved by providing a multiplate clutch between the one-way clutch-assembly and the take-up means. By overcoming friction in the multiplate clutch, the take-up means can be rotated back to its original position.
Because it is necessary to be able to rotate the take-up means in one direction to take up play during operation of the braking system and in the opposite direction to release the adjuster assembly when new linings are to be fitted, a risk exists that the take-up means may be caused to rotate in the xe2x80x9creleasexe2x80x9d direction, for example due to vibration, even when it is not intended to replace the linings. If this were to happen, the stroke of the brake lever would initially be longer than intended, though would adopt its operating length after several applications of the brakes.
The present invention in its several disclosed embodiments alleviates the drawbacks described above with respect to brake lining wear adjuster assemblies. It is therefore an object of the present invention to provide an adjuster assembly in which the risk of improper operation is reduced at the same time that servicing of the brake unit is not impaired. The specific structures through which these benefits are delivered will be described in detail hereinbelow.